The invention relates to power factor correction in switching power conversion.
Power factor correction techniques are used to increase the efficiency of conversion from an AC source to a DC load. Power factor correction reduces harmonic currents, thereby reducing the peak current demanded from the AC source. Non-fundamental harmonic currents do not contribute to power flow, but circulate in power distribution systems causing I.sup.2 R losses. In Y-connected three-phase systems, harmonically pure currents of equal amplitude add to zero in the neutral connector. Harmonically rich currents, however, can add to a value that is higher than any individual phase current. Power factor correction attempts to assure that current is drawn from the AC source in amounts and at times that are proportional to and in phase with the usual sinusoidal variations in the voltage at the AC source.
One known way to achieve both power factor correction and a step down to a low DC load voltage is to cascade two power conversion stages. The circuit shown in FIG. 1 includes a first boost converter stage 1 that rectifies a bipolar input voltage from an AC source 12 in a bridge rectifier 2 and then boosts the voltage in an active current-shaping circuit 3. Power factor correction is achieved by a circuit 4 that includes a switch 5. The high DC voltage produced by the current-shaping circuit 3 is transformed by a fast-regulating DC-to-DC converter stage 6 that produces a usually lower voltage, isolated DC output voltage 7. In converter 6, energy is transferred through a transformer 8 in discrete packets that are defined by the opening and closing of a switch 9. Switch 9 is controlled by a feedback circuit 11 to maintain a regulated output voltage in the face of shifting source and load levels.
Attempts have been made to provide power factor correction in a simpler single power conversion stage. In such single-stage converters, the swing between the maximum and minimum values of the DC input voltage to the DC-to-DC isolating converter may be large. This voltage appears across a bulk capacitor, like capacitor 20 of FIG. 1. Variations in the AC source voltage and the load on the DC-to-DC converter can cause the bulk capacitor voltage 40 to have a swing of over 3:1. The DC-to-DC converter must operate over at least this same swing, which reduces the possibility for achieving an optimum design of the DC-to-DC power conversion elements. Large variations in the bulk capacitor voltage require the capacitor to handle large ripple currents. A 3-to-1 swing in the bulk capacitor voltage requires the capacitor to have a ripple current rating three times higher than a capacitor operating at high-end voltage for all input conditions.
The variation in the bulk capacitor voltage may be limited using a discontinuous conduction mode (DCM) boost AC-to-DC converter connected to a downstream DCM Flyback circuit. The bulk capacitor voltage tracks variations in AC input voltage to the AC-to-DC converter. If a DC source is used to drive the bulk capacitor, then the bulk capacitor voltage will track variations in the DC supply voltage. In either case, the DC-to-DC converter has to handle at least the range of variations of the bulk capacitor voltage.